Please elaborate. I don't see how having a library of SASS mixins that you mix in to make specific CSS classes for specific purposes make anything harder to manage? If anything, decomposing things to smallest common denominator makes it easier to build better and more versatile CSS for your apps.

It's not the sass mixins that makes things harder to manage. The problem is having nested CSS rules that cause your CSS to become tightly coupled to the structure of HTML. Then when the HTML changes, you have to rethink the relationships. And if you're not using nesting, then isn't what you described just utility classes abstracted one level away?